IMO there's nothing in the above posts that doesn't ring true.
Particularly, they are bright and do produce a lot of light. However, they don't pretend to replace the long range beamed lighting that you might otherwise get from some expensive good quality spot lamps.
I have a 17" one under the roof rack and a 24" between the headlights. Never use them with any oncoming traffic, irrespective of distance, that really wouldn't be good practice and fully justified road rage may be the outcome.
Don't even use them if there's a vehicle ahead of you going in your direction, it would be very ditracting for them, and may cause the same justified reaction.
But on your own, in the dark, they are great. You get a big pool of light in front of you, so for offroad or lonely late night driving, they are worth their weight.